The offset lithographic printing process utilizes a developed planographic printing plate having oleophilic image areas and hydrophilic non-image areas. The plate is commonly dampened before or during inking with an oil-based ink composition. The damping process utilizes an aqueous fountain solution such as those described in U.S. Pat. Nos. 3,877,372, 4,278,467 and 4,854,969. When water is applied to the plate, the water will form a film on the hydrophilic areas (i.e. the non-image areas of the plate) but will contract into tiny droplets on the oleophilic plate areas (i.e. the image areas). When a roller carrying an oil-based ink composition is passed over the dampened plate, it will be unable to ink the areas covered by the aqueous film (the non-image areas), but will emulsify the water droplets on the water repellant areas (the image areas) which will then take up ink. The resulting ink image is transferred ("offset") onto a rubber blanket, which is then used to print a substrate such as paper.
It has been proposed to apply "direct" ink jet printing techniques to lithographic printing. For example, European Patent Publication No. 503,621 discloses a direct lithographic plate making method which includes jetting a photocuring ink onto the plate substrate, and exposing the plate to UV radiation to harden the image area. An oil-based ink may then be transferred to the image area for printing onto a printing medium. However, there is no disclosure of the resolution of ink drops jetted onto the substrate, or the durability of the lithographic printing plate with respect to printing runlength.
It is known to improve the resolution of ink jet printers by applying an ink receiving layer to substrates such as metal, plastic, rubber, fabrics, leather, glass and ceramics, prior to printing thereon. See, for example, European Patent Publication No. 738,608 which discloses a thermally curable ink receiving layer containing a first water soluble high molecular weight compound having a cationic site in the main polymer chain and a second water soluble high molecular compound having a side chain containing a condensable functional site. Alternatively, the second high molecular weight compound may be replaced with a monomer or oligomer having at least two (meth)acryloyl sites, which results in a UV radiation curable ink receiving layer.
The addition of fluorosurfactants to an ink receiving layer in order to enhance its surface smoothness, image quality or coating characteristics is known. Thus, U.S. Pat. No. 5,084,340 discloses a transparent ink jet receiving element having an ink receptive layer that contains a fluorosurfactant in order to provide enhanced surface smoothness and to facilitate coating of the ink receiving layer onto the support. European Patent Publication No. 742,109 describes a recording sheet containing a binder, and a mixture of an anionic and a cationic fluorosurfactant which is said to increase dot size and improve image quality.
U.S. Pat. No. 4,781,985 discloses an ink jet transparency which comprises a substantially transparent support and a substantially clear coating thereon which includes a specific fluorosurfactant to prevent trailing of the ink jet ink. The coating thickness is 2-15 microns. The fluorosurfactant is said to result in much larger dot sizes and an improved apparent projected density.
Application of a continuous surfactant layer to a conventional lithographic printing plate substrate for use in conjunction with a photosensitive overcoat is also known. U.S. Pat. No. 5,900,345 discloses a lithographic printing plate having a radiation sensitive coating. A hydrophilizing layer between the substrate and the radiation sensitive coating may additionally contain a surfactant to improve the surface smoothness of the subsequently applied radiation sensitive coating. Alternatively, a separate surfactant layer may be coated on the hydrophilizing layer.
An object of the present invention is to provide a printing plate precursor which can be used to prepare a lithographic printing plate having improved resolution.
A feature of the present invention is a printing plate substrate on which a surfactant is adsorbed prior to being imaged with an ink receptive composition.
Another feature of the invention is the removal of surfactant from non-image areas of the imaged printing plate prior to use of the printing plate on the printing press.
An advantage of the present invention is improved resolution of an ink receptive composition when it is applied by an ink jet printing apparatus without unacceptable reduction of adhesion of the ink receptive layer to the printing plate precursor.